And to underline the message for the future of fossil fuels: The owners of Arizona’s Navajo Generating Station, the largest coal-fired plant in the west of the U.S. have announced that they consider closing the plant, because low natural gas prices and the rising costs of generating electricity using coal make it too expensive to operate it. Coal-fired power plants, once the workhorse of the U.S. electricity grid, are the biggest individual climate polluters in the country, and they’re falling victim to coal’s biggest competitor: plentiful, cheap natural gas.